The recycling of waste paper represents a large fraction in paper production, and the presence of impurities or contaminants mixed with the waste paper, for instance bulky or metallic objects, plastics, etc . . . , gives rise to important problems, particularly at the level of the discharge valves of the devices located in the first processing stages where waste paper is converted into a coarse pulp.
The coarser impurities are eliminated during these first stages, generally through processes which operate discontinuously, product flow being controlled by means of valves: for instance, the valve is opened for allowing impurities to flow out, and is thereafter closed back for carrying out a new processing cycle.
These valves are usually of the slide-valve type called "shovel sluice" or "traversing scoop" or "guillotine" which allows for a quick opening or closing of the duct being controlled. However, when contaminants are evacuated from an apparatus, either for being rejected to the exterior or else for being washed in another apparatus, some of them will become lodged inside the valve slide or else will become jammed in the opening passage of the valve (metal rods or other bulky items), or again they will distort the tip of the "shovel" at the moment of closing, thus preventing it from operating correctly afterwards. Also, salts which are abundantly present in processing water give rise to an important corrosion of the "scoops", even when these are made of stainless steel, so that they become jammed or seized. These breakdowns and damages result in stoppages in the operation of the plant, with important losses of products and severe pollution problems.